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Dermatology Update: Can dextromethorphan be used to treat repetitive itching and scratching in atopic dogs?

Atopic dermatitis is one of the most common causes of pruritus in dogs. It is diagnosed based on a dog's history and clinical signs and the elimination of other causes of pruritus.

The pathogenesis of pruritus in dogs is still not well understood. To further complicate matters, some clinicians speculate that certain dogs have a combination of sensory irritation (true itch) and repetitive behaviors (self-directed itch), though these dogs are hard to differentiate from dogs with purely sensory irritation. But the dogs that are most suspected of having this combination are those that clients describe as becoming itchy when stressed.

Treatment options for atopy vary depending on a variety of factors, including the severity of signs, whether the signs are year-round or seasonal, the dog's response to medical treatment, and the owner's financial constraints. Dogs receiving immunotherapy often also receive concurrent antipruritic therapy on either a long-term or intermittent basis.

In a recent study, oral dextromethorphan hydrobromide was evaluated in 14 dogs with atopic dermatitis to determine whether the drug had any effect on repetitive behaviors associated with or suggestive of pruritus (e.g. self-licking, self-chewing, self-biting).1 A veterinary dermatologist diagnosed atopic dermatitis in the dogs, and all of the dogs were free of other complicating causes of pruritus (e.g. mites, yeast). After a three-week washout period from any drugs the dogs may have been receiving before the study, the dogs were randomly enrolled in a four-week, placebo-controlled, double blind crossover study. The dogs received 2 mg/kg dextromethorphan in a gelatin capsule every 12 hours and a placebo for two weeks each. During the study, oatmeal soaks were allowed, but no other therapies. Twelve dogs completed the study; two dropped out because of adverse effects (sedation, diarrhea). Because of a lack of compliance with data collection, the investigators were only able to evaluate data from 10 dogs. An important component of the study was the owners' scoring of their dogs' pruritus. The owners were required to record how much time they spent with their dogs and the amount of time the dogs exhibited certain itch behaviors. In addition, a dermatologist examined the dogs at various points in the study.

The investigators used a calculation called the itch percent to evaluate the data. The itch percent was calculated based on the time the owners spent with their dogs and how much time the dogs were observed to be involved in itch-scratch behaviors during these periods. Dogs receiving the placebo were observed to exhibit itch behaviors 8.7% of the time; dogs receiving dextromethorphan exhibited itch behaviors 6% of the time. The difference (2.7%) was statistically significant and represents a 31% decrease in observed itch behaviors. A global dermatology score was calculated for all the dogs, which consisted of three components of the dermatologist's assessment (a pruritus score, an inflammation score, and an overall score). The dermatologist's global assessment was more favorable after the active treatment phase in 11 of the 12 dogs, and the pruritus score during dextromethorphan treatment was significantly less compared with the placebo and baseline. The investigators concluded from these findings that dextromethorphan was beneficial in reducing the time the dogs spent licking, chewing, and self-biting.

Commentary

The findings in this study suggest that dextromethorphan may be clinically useful in atopic dogs with habituated pruritic behaviors. However, a placebo effect may still have been a factor. Moreover, the investigators stated that the oatmeal soaks might have contributed to some of the overall gross improvement in the skin. And the 31% decrease in pruritus represents 23 minutes of itch-scratch behavior when the dogs received the placebo vs. 17 minutes when they received dextromethorphan. I'm not sure that the average client would find a decrease of six minutes of itching to be a great improvement. Finally, in a recent study on the pharmacokinetics of dextromethorphan after intravenous and oral administration in healthy dogs, the investigators found that the drug had a short half-life, was rapidly cleared, and had poor bioavailability.2 The authors of this pharmacokinetics study concluded that the drug's erratic absorption, short elimination half-life, and rapid clearance limit its potential usefulness when administered orally long-term.2